digraph G {
  a[label="", shape=rectangle];
  b[label="", shape=rectangle];
  c[label="", shape=rectangle];
  {rank=same; b,c}
  d[label="sort [4]", shape=rectangle];
  e[label="sort [2]", shape=rectangle];
  f[label="sort [6]", shape=rectangle];
  g[label="sort [1]", shape=rectangle];
  {rank=same; d,e,f,g};
  a -> b;
  a -> c;
  b -> d;
  b -> e;
  c -> f;
  c -> g;
}
